FIGURE 1.

Experimental fish trials to test phage delivery by feed (A), by bath (B), and by intraperitoneal injection (C). (A) Eight hundred and ninety-five rainbow trout fry were divided randomly in 12 aquaria (55 fish ± 4) which are represented by colored circles. Fish were fed at 2% of their body weight with either control feed (in gray), phage-immobilized (in blue), or phage-sprayed (in red) feed. Bacteriophage drawings indicate FpV4 and FPSV-D22, which were administered to fish by feed. For each group, three of the four aquaria were challenged with F. psychrophilum (1 × 10⁴ CFU fish^–1; indicated by a syringe with yellow content because of the coloration of the bacteria). Fish in the fourth aquarium were injected with sterile TYES-B as control for the infection. Two of the bacterial challenged aquaria per group were used to follow mortality. The remaining aquaria were dedicated to sampling (five fish at sampling point). (B) Hundred and twenty-five rainbow trout fry were divided in four aquaria (circles; 31 fish ± 1 per aquarium) and challenged with F. psychrophilum (1 × 10⁵ CFU fish^–1; syringe with yellow content). Fish in two aquaria were exposed to three rounds of FpV4 and FPSV-D22 phages bath (blue circles; indicated by bacteriophage drawings). Mortality was followed w = week. (C) Hundred and twenty rainbow trout (7 g), divided in six aquaria (circles, 20 fish ± 0 per aquarium), were IP challenged with F. psychrophilum (1.7 × 10⁷ CFU fish^–1; syringe with yellow content). Three days later, fish in three aquaria were exposed to phages FpV4 and FPSV-D22 by IP injection (blue circles; indicated by bacteriophage drawings). This aquaria were used to follow mortality. One additional aquarium was included for few sampling where fish were only exposed to the two component phage mix (light blue circle; 15 fish). Created with BioRender.com (the figure was exported under a paid subscription).